As examples of an optical recording medium (hereinafter referred to as “optical disk”) having an information recording layer formed on a surface of light-incident side, and a transparent resin covering the information recording layer, e.g. CDs and DVDs are widely used. Further, in an optical head device for writing and/or reading an information to/from DVDs, one employing a laser diode of 660 nm wavelength band as a light source and an objective lens having a NA (numerical aperture) of from 0.6 to 0.65, are known.
Heretofore, a commonly used DVD (hereinafter referred to as “single layer optical disk”), has a single information recording layer and a cover layer of 0.6 mm thick. However, in recent years, in order to increase information amount in each optical disk, a read-only type or a readable-writable type optical disk having two information recording layers (hereinafter referred to as “double layer optical disk”) has been developed.
Thus in a case of writing and/or reading to/from a double layer optical disk by using an optical head device having an objective lens optimally designed to have zero aberration for a single layer optical disk, when a cover thickness is different, a spherical aberration is generated according to the difference of the cover thickness and convergence of incident light to an information recording layer is deteriorated. In particular, in a case of double-layer optical disk of writing type, deterioration of convergence corresponds to decrease of converging power density, which causes a writing error, such being a problem.
To cope with this problem, in recent years, in order to improve recording density of an optical disk, an optical disk having a cover thickness of 0.1 mm (hereinafter it is also referred to as “high density optical disk”) is also proposed. Further, an optical head device for writing an information to such an optical disk, employs a laser diode emitting laser light of 405 nm wavelength band and an objective lens having a NA of 0.85. However, also in this case, with respect to a double layer optical disk of recording type, a spherical aberration generated corresponding to the difference of the cover thickness, causes a writing error, such being a problem.
As means for correcting a spherical aberration caused by the difference of the cover thickness of e.g. the above-mentioned double layer optical disk, a method of employing movable lens group or a liquid crystal lens, has been known.
(I) For example, in order to carry out correction of spherical aberration by using a movable lens group, an optical head device 100 shown in FIG. 8 for writing and/or reading an optical disk D, has been proposed (for example, JP-A-2003-115127).
The optical head device 100 comprises a light source 110, an optical system 120 of various types, a photo-acceptance element 130, a control circuit 140 and a modulation/demodulation circuit 150, and further, a first and a second movable lens groups 160 and 170. Further, the first movable lens group 160 includes a concave lens 161, a convex lens 162 and an actuator 163, which exhibits a focal-length variable lens function that a power of the movable lens group 160 is continuously changeable from positive (convex lens) to negative (concave lens) by moving the convex lens 162 fixed to the actuator 163 in an optical axis direction. By disposing the movable lens group 160 in an optical path to an optical disk D, it becomes possible to correct a spherical aberration containing a power component and to adjust a focal point of incident light to an information recording layer (not illustrated) of the optical disk D having a different cover thickness.
However, in the case of employing the movable lens group 160, there has been a problem that the size of the optical head device 100 becomes larger since the pair of lenses 161 and 162 and the actuator 163 are required, and the mechanical design for the movement becomes complicated.
(II) Further, in order to correct a spherical aberration caused by the difference of cover thickness of an optical disk, an optical head device employing a is liquid crystal lens 200 as shown in FIG. 9, has been proposed (for example, JP-A-5-205282).
The liquid crystal lens 200 has a construction that it comprises a substrate 230 having a flat surface on which a transparent electrode 210 and an alignment film 220 are formed, a substrate 260 having a curved surface symmetric about an axis and having a surface shape S(r) represented by the following formula being a sum of powers of a radius r, on which a transparent electrode 240 and an alignment film 250 are formed, and a nematic liquid crystal 270 sandwiched by the substrates 230 and 260.
In the liquid crystal lens 200, when a voltage is applied between the transparent electrodes 210 and 240, alignment of molecules of the liquid crystal 270 changes and the refractive index of the liquid crystal 270 changes. As a result, a wavefront of transmission light changes in accordance with refractive index difference between the substrate 260 and the liquid crystal 270.
Here, the refractive index of the substrate 260 equals to the refractive index of the liquid crystal 270 when no voltage is applied. Accordingly, when no voltage is applied, transmission wavefront is not changed from that of incident light. On the other hand, when a voltage is applied between the transparent electrodes 210 and 240, a refractive index difference Δn is generated between the substrate 260 and the liquid crystal 270, and phase difference of transmission light corresponding to Δn·S(r) is generated (refer to Formula (1) for S(r)). Accordingly, it is possible to correct an aberration by fabricating the surface shape S(r) of the substrate 260 so as to correct a spherical aberration caused by the difference of cover thickness of an optical disk D, and by adjusting the refractive index difference Δn according to applied voltage.S(r)=α1r2+α2r4+α3r6+ . . .   (1)
wherein r2=x2+y2 
However, in the case of liquid crystal lens described in FIG. 9, since the refractive index change of the liquid crystal 270 in response to applied voltage is at most about 0.3, it is necessary to increase the concave-convex height of S(r) to generate a large phase difference distribution Δn·S(r) corresponding to a power component for changing the position of a focal point of incident light. As a result, the layer of liquid crystal 270 becomes thicker, which causes problems that driving voltage increases and response becomes slower.
To cope with this problem, in order to reduce thickness of liquid crystal layer, it is effective to correct only spherical aberration requiring minimum amount of aberration correction, except for power component. However, when the substrate 260 is fabricated to have a surface shape S(r) so as to correct only spherical aberration, if the optical axis of an objective lens for converging incident light on an information recording layer of an optical disk, and the optical axis of the liquid crystal lens are not aligned to each other, a coma aberration is generated which causes a problem that convergence to the information recording layer is deteriorated and writing or reading is prevented.
(III) By the way, in order to develop a substantial lens function capable of changing also a power component corresponding to change of the position of focal point of incident light without increasing the thickness of liquid crystal layer, a liquid crystal diffraction lens 300 shown in FIG. 10 is also proposed (for example, JP-A-9-189892).
In the liquid crystal diffraction lens 300, a transparent electrode 320 is formed on one side of a substrate 310 on which a predetermined saw-tooth-shaped relief is formed, and the transparent electrode 320 and an opposing electrode 330 sandwich a liquid crystal layer 340. When a voltage is applied between the electrodes 320 and 330, substantial refractive index of the liquid crystal layer 340 for extraordinarily polarized light changes from an extraordinary refractive index ne to an ordinary refractive index no. Here, “substantial refractive index” means an average refractive index in the thickness direction of the liquid crystal layer.
Provided that the refractive index of the substrate 310 having the saw-tooth-shaped relief structure is designated as nF, and the wavelength of incident light is designated as λ, by forming the saw-tooth-shaped relief grooves so as to have a depth d satisfying an equation d=λ/(ne−nF), maximum diffracting efficiency is obtained at the wavelength λ when no voltage is applied, and thus, a diffraction lens is formed. Further, even if the wavelength λ of incident light is changed, application voltage can be adjusted so as to produce the maximum diffraction at the wavelength λ.
In the liquid crystal diffraction lens 300 having such a construction, since it is only necessary to fill the grooves of the saw-tooth-shaped relief with the liquid crystal layer 340, 4the liquid crystal layer 340 can be thinner than the liquid crystal 270 shown in FIG. 9 which is a type of liquid crystal to be used for the above-mentioned liquid crystal lens 200 to correct spherical aberration containing a power component.
However, in the liquid crystal diffraction lens 300, since the transparent electrode 320 is formed on the saw-tooth-shaped relief surface, it is necessary to satisfy a relation no<nF<ne to obtain power components of both positive and negative. In this case, since no≠nF, a fixed phase difference represented by a formula φ=d×(nF−no)λ is generated for ordinarily polarized light, which has been a problem in a case of applying the liquid crystal lens to an optical head device employing a polarization optical system.
(IV) In order to obtain power components of both positive and negative for ordinarily polarized light without changing transmission wavefront, a liquid crystal diffraction lens element 400 as shown in FIG. 11 is considered. In the liquid crystal diffraction lens element 400, a liquid crystal layer 414 filling a cell constituted by a pair of transparent substrates 411 and 412 and a seal 413, is driven by transparent electrodes 415 and 416 formed on the transparent substrates 411 and 412. On a surface of the transparent electrode 415, a Fresnel lens surface 417 being a saw-tooth shaped relief surface is formed. In the liquid crystal diffraction lens element 400, since nF=no, transmission wavefront does not change for ordinarily polarized light. Further, since distribution of substantial refractive index is formed in the liquid crystal layer 414 according to specific dielectric constant of a material constituting the Fresnel lens surface 417, it is possible to generate power components of both positive and negative according to the magnitude of applied voltage.
However, when a voltage producing 0-th order light having no power change, is applied, since the Fresnel lens surface 417 is disposed between the liquid crystal layer 414 and the transparent electrode 415, a voltage applied to the liquid crystal layer 414 is distributed according to the shape of the Fresnel lens surface 417, and a phase difference is generated. As a result, a problem that diffraction efficiency of 0-th order light decreases, is generated.